The vehicle code, section 23152(b) makes it unlawful to drive with “0.08 percent or more, by weight, of alcohol in [a driver’s] blood.” You might immediately note that most DUIs are charged based on a breathalyzer measurement, not a blood measurement. I previously discussed how breathalyzers measure the alcohol content in a person’s blood but to complicate matters, the alcohol weight in a person’s breath must be translated into an equivalent blood weight measurement . If you continue reading section 23152(b) it states, “percent, by weight, of alcohol in a person’s blood is based upon grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 liters of breath.”

These different measurements of alcohol by weight depending on whether the test was of blood or breath is known as “partition ratio variability.” The more accurate measurement of BAC is by measurement of alcohol content in the blood. Breath analysis loses some accuracy and though rigorous scientific testing, test established that the average amount of alcohol in 230 liters of breath is equivalent to what would be found in the 100 milliliters of the same subject’s blood. This may vary from person to person on either side of the scale. To allow for possible variations among individuals and to give the benefit of the doubt to the driver, the California legislature, set the conversion at 210 liters of breath to 100 milliliters of blood, or a ratio of 2,100 to one.

Before it was amended in 1990, section 23152(b) defined blood alcohol concentration (BAC) only in terms of “grams of alcohol per 100 milliliters of blood.” Because, then as now, many DUI charges were based on breathalyzer results, the breath results had to be converted using the standard 2,100 to one ratio. But because the partition ratio was not written into the statute, a common DUI defense was based on the effect of partition ratio variability factors. Following amendment of the law, the California Supreme Court held that the amended language was meant to criminalize driving with the specified blood alcohol level or the specified breath-alcohol level. In other words, the amendment did not simply provide an alternate method for calculating BAC but was meant to make it unlawful to drive with an .08 percent or more BAC as measured per 210 liters of breath.

Some years later, in 2013 to be exact, the California Supreme Court took up another challenge to the breathalyzer measurement of BAC. In the years prior to this case, defendants occasionally challenged the scientific accuracy of breathalyzer measurements in jury trials. Several well-known expert witnesses worked this circuit so to speak. These scientific experts would testify that numerous studies have shown that even properly working and fully calibrated breathalyzer devices are often inaccurate. The 2013 case that ended up before our state’s Supreme Court, People v. Vangelder, concerned the admissibility of expert testimony concerning the general accuracy or reliability of breathalyzer devices. Harkening back to the 1990 amendment to section 23152(b), the court held that the testimony is irrelevant and hence, inadmissible, under the per se DUI statute. Simply stated, the court held that even if the breathalyzers may produce inaccurate results among individuals, it does not matter because “the statutorily proscribed amount of alcohol in expired breath corresponds to the statutorily proscribed amount of alcohol in blood, as established by the per se statute.”

While the Supreme Court’s decision explains much more than that, it is a complicated case and for purposes here, it is enough to say that DUI defendants can no longer argue that breathalyzer tests are subject to scientific question. However, it is important to note that the Supreme Court did not foreclose challenging the accuracy of a particular breathalyzer device when there is reason to suspect it was improperly used, improperly calibrated, or otherwise defective.

Orange County DUI defense attorney, William Weinberg, makes it his business to explore every possible defense to a DUI or DUI-related charge. He offers a free consultation regarding your DUI matter. If you have been charged with a DUI or a related charge, contact Attorney Weinberg at 949-474-8008 or email him at and he will be happy to go over your defense options.